As Zika escalates into a public health crisis and the number of mosquito-transmitted cases grows, Republicans and Democrats are pointing fingers over the failure by Congress to commit federal dollars to fight the virus.

Health and Human Services Secretary Sylvia Burwell warned lawmakers on Wednesday that her Zika budget is running out quickly. Without more money soon, she said, the "nation's ability to effectively respond to Zika will be impaired."

Yet lawmakers left Washington in mid-July for a seven-week recess without approving any of the $1.9 billion requested by President Barack Obama in February to develop a vaccine and control the mosquitoes that carry the virus.

Republicans angered Democrats by adding a provision to a $1.1 billion take-it-or-leave-it measure that would have blocked Planned Parenthood clinics in Puerto Rico from receiving money.

House Speaker Paul Ryan, writing in USA Today on Thursday, did not mention that provision in blaming Democrats for blocking the legislation. Their obstruction, according to Ryan, R-Wis., is "a blatant ploy in an election year."

But McConnell has signaled he is in no rush to return. Writing in the Lexington (Ky.) Herald-Leader on Monday, he criticized Democrats for balking at passing the bill. He said they'll get another chance after Labor Day when Congress is back in session.

Some important points to know about the issue:

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Burwell's letter seeks to counter Republicans who've criticized the Obama administration for not using several hundred million dollars already in the budget for Zika prevention. The money was initially allotted for fighting Ebola but was redirected to address Zika.

Sen. Marco Rubio, R-Fla., said Tuesday that's there's no excuse for not spending money that Congress has provided. "Why are they holding that money back?" he asked.

Burwell said her agency is committed to using "scarce federal dollars aggressively and prudently." The Centers for Disease Control received the bulk of the $374 million "repurposed" for Zika domestic response efforts, she said, and it will exhaust the remainder of the money by Sept. 30.

Money for vaccine development will run out even sooner, she said. The second phase of clinical trials would be delayed as a result, and Americans would have to wait longer for a vaccine, according to Burwell.

"Now that the United States is in the height of mosquito season and with the progress in developing a Zika vaccine, the need for additional resources is critical," Burwell wrote.

Sen. Bill Nelson, D-Fla., said Congress does not have to interrupt its break to pass the bill. Republicans quickly dismissed his proposal.

Nelson's state has become the epicenter for Zika in the U.S. At least 15 people are reported to be infected with the virus in Miami's Wynwood arts district. These are believed to be the first mosquito-transmitted cases in the mainland United States.

Nelson said in a letter Tuesday to McConnell that the Senate could pass a bill through a procedure known as a pro forma session that requires the presence of only a few senators.

But even Nelson is not optimistic that will happen, and he took a jab at McConnell, predicting the Senate would move swiftly if a transmitted Zika case was reported in Kentucky.

Don Stewart, McConnell's spokesman, said Nelson's proposal was not plausible unless Democrats were willing to end their filibuster of a Zika bill the House has passed. Otherwise, the Senate would be only approving an earlier version of the legislation that Obama could not sign into law, Stewart said.

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Zika is a looming economic development problem too, according to Rubio. Many Florida businesses depend heavily on tourism and the state's economy could be hurt if potential visitors decide to stay away, he said.

"I can foresee now when people that are planning to come to Florida, to go fishing perhaps, will decide to cancel their trip because they're worried about mosquitoes and they're worried about Zika," Rubio said.

White House spokesman Josh Earnest said Wednesday that it's up to Congress to pass the legislation so that more can be done.